1. Field of the Invention
The present invention relates to a detection element. In particular, the present invention relates to a detection element that accumulates charges generated due to radiation being irradiated and detects an accumulated charge amount as information expressing an image.
2. Description of the Related Art
A detection element, such as an FPD (flat panel detector) or the like, in which an X-ray sensitive layer is disposed on a TFT (thin film transistor) active matrix substrate, and that can convert X-ray information directly to digital data, has been put into practice in recent years. As compared with a conventional imaging plate, an image can be confirmed immediately at an FPD. Further, the FPD has the advantage that video images as well can be confirmed. Therefore, the popularization of FPDs has advanced rapidly.
Various types of detection elements are proposed. For example, there is a direct-conversion-type detection element that converts radiation directly into charges in a semiconductor layer, and accumulates the charges. Moreover, there is an indirect-conversion-type detection element that once converts radiation into light at a scintillator of CsI:T1, GOS (Gd2O2S:Tb), or the like, and, at semiconductor layer, converts the converted light into charges and accumulates the charges.
In this kind of detection element, an electrode (hereinafter referred to as a “bias electrode”) that applies a bias voltage to one surface of the semiconductor layer and an electrode (hereinafter referred to as a “collecting electrode”) that collects charges at the other surface of the semiconductor layer are provided. In this kind of detection element, charges generated at the semiconductor layer are collected at the collecting electrode and are accumulated as information expressing an image.
In this kind of detection element, a voltage level of the collecting electrode changes in accordance with a charge amount of the accumulated charges.
For example, in a case where a positive bias voltage is applied from the bias electrode to the semiconductor layer, negative charges that have been generated at the semiconductor layer moves to the bias electrode. Further, positive charges that have been generated at the semiconductor layer moves to the collecting electrode.
For this reason, the larger the amount of charges that have been collected and accumulated becomes, the more a voltage level of the collecting electrode increases. As a result, a potential difference between the bias electrode and the collecting electrode decreases.
In this kind of detection electrode, when the potential difference between the bias electrode and the collecting electrode decreases, charges generated at the semiconductor layer are trapped by a defect level at the semiconductor layer interior and remain therein without moving. Accordingly, in a case where it is attempted to continuously read out images to obtain video images, a residual image may be generated.
In particular, in an indirect-conversion-type detection element, amorphous silicon or the like is used as the semiconductor layer. In the indirect-conversion-type detection element, the semiconductor layer is thin, and a bias voltage that is applied to the semiconductor layer is generally as low as from several volts to several tens of volts. As a result, in the indirect-conversion-type detection element, such a phenomenon may be easily generated.
As a technique for suppressing generation of a residual image, Japanese Patent Application Laid-Open (JP-A) No. 2001-345440 discloses a technique of providing a TFT switch for resetting for each of the respective pixels of a detection element, and driving the TFT switches for resetting immediately before X-rays are irradiated, to eliminate charges remaining within the respective pixels.
Further, Proc. SPIE, Vol. 4682, pp. 820-821 discloses a technique of making it difficult to recognize a residual image, by illuminating light from a backlight to a detection element immediately before X-rays are irradiated, and evenly filling in defect levels at the interior of a semiconductor layer with charges to reduce variation in the remanence of charges per pixel.
However, in the techniques disclosed in JP-A No. 2001-345440 and Proc. SPIE, Vol. 4682, pp. 820-821, it is necessary to drive the TFT switches for resetting or illuminate light from the backlight, immediately before X-rays are irradiated. Accordingly, in cases where it has been attempted to continuously read out radiographic images from a detection element to obtain video images, there has been a limit to the improvement of a frame rate.
Further, the techniques disclosed in JP-A No. 2001-345440 and Proc. SPIE, Vol. 4682, pp. 820-821, are techniques for suppressing the influence of a residual image that has been generated. Accordingly, these techniques cannot suppress the generation, per se, of a residual image.